Platelet-derived endothelial cell growth factor (PD-ECGF), identical to thymidine phosphorylase, has been reported as an angiogenic factor in human malignancies. However, the role of PD-ECGF in human hepatocellular carcinoma (HCC) is still unconfirmed. Herein, we studied the expression of PD-ECGF in 27 human HCC cases by immunohistochemistry, to clarify the relationship to tumor angiogenesis. The immunoreaction of PD-ECGF in HCC cells was scored in both the staining percentage and intensity. CD34, an endothelial cell marker, was used to evaluate the intratumoral microvessel density (IMVD). PD-ECGF expression was noted in carcinoma cells in 14 (51.9%) of 27 HCCs. In these cases, the carcinoma cells showed heterogeneous staining in both the nucleus and cytoplasm. Tumor-associated stroma cells and infiltrating lymphocytes were also stained. Kupffer cells in non-tumor areas were strongly positive. Statistically, the expression of PD-ECGF increased in HCC specimens with high Edmondson grades (III-IV) or portal vein tumor thrombosis (PVTT) (P<0.05). Additionally, the IMVD of PD-ECGF-positive HCC specimens (136.071+/-31.008, mean +/- SD) was higher than that of the PD-ECGF-negative HCC specimens (61.077+/-15.795) (P<0.05). These findings may suggest that PD-ECGF is one of the angiogenic factors in human HCCs. Furthermore, with the increasing expression of PD-ECGF, HCC cells show poor differentiation and invasive behavior.